Field of the Invention
The present invention generally relates to an apparatus and a method for measuring as well as controlling the water content of water-containing liquid mixture contained in a tank.
In particular, the present invention relates to an apparatus and a method for the in-situ control of the water content in a so-called hot phosphoric acid (HOT PHOS) baths.
Usually, in the manufacture of semiconductor integrated circuits, HOT PHOS baths are used for wet-etching of silicon nitride. Silicon nitride is generally used as a masking layer. When it becomes necessary to remove the silicon nitride mask, normally, wet etching with hot phosphoric acid at 160° C. with approximately 85 wt-% H3PO4 is used. Because the etching rate of silicon nitride is more than ten times higher than the etching rate of silicon dioxide, silicon nitride is selectively etched. However, two important factors influence the etching rate of both silicon nitride and silicon dioxide: 1) the bath temperature that is normally a constant temperature at around 160° C., and 2) the water content of the bath.
The water content of the bath varies because of the continuous water evaporation due to the high temperature, the water evaporation during lid opening, and the chemical reaction as indicated above. So far, a constant amount of water is continuously spiked during the process, but such a procedure is not well controlled and the amount of spiked water is often determined empirically.
To maintain constant the water content in a phosphoric acid/water system used for silicon nitride etching, U.S. Pat. No. 5,779,927 to Lo suggests an apparatus and a method in which the liquid acid evaporant is condensed and returned to the main volume of the acid. To control the amount of pure water added, either the pH of the condensed evaporant is measured or, alternatively, the conductivity of the phosphoric acid/water system is measured.
However, these measurement mechanisms involve major structural changes of the etching apparatus. In particular, a special piping has to be attached to the apparatus to return the condensate through the pH meter to the acid bearing container.